1. Field of the Invention
The present invention relates to a radio transmitter apparatus, more particularly relates to an automatic level control circuit (hereinafter also referred to as an ALC circuit) in a radio transmitter apparatus having an automatic power control (hereinafter also referred to as an ATPC) function and, in more detail, relates to a detection unit in an ALC circuit
In a radio transmitter apparatus, an ALC circuit is generally provided in order to hold an output power at a predetermined constant level. Namely, a detection stage forming part of the ALC circuit detects a transmitting output level from the radio transmitter apparatus as a voltage and applies feedback so as to change the gain at a transmission power generating stage in the radio transmitter apparatus according to the change in level of the detected voltage. In order to change the gain, usually an attenuation amount or gain of a variable attenuator or a variable gain amplifier constituting part of the transmission power generating stage is controlled.
A radio-transmitter apparatus provided with such an ALC circuit, however, is further being given the above ATPC function. The role of this ATPC function is as follows.
For example, when transmitting and receiving radio frequency (RF) signals between one repeater office and another repeater office via antennas of the offices, the radio system is ordinarily designed so that stable transmission and reception of the radio frequency signal are maintained even when the communications conditions become the worst due to the meteorological conditions or topological conditions between these two repeater offices.
However, the probability of the communications conditions becoming the worst due to rain, snow, fading, etc. is very low. Experience has taught that good communications conditions are secured in most hours. Nevertheless, radio systems are being designed assuming the radio signals are transmitted and received under the worst communications conditions. For this reason, the radio transmitter apparatus consumes more than the necessary electric power in most hours. This is becoming a constant obstacle in satisfying demands for a reduction of the power consumption of the radio transmitter apparatus and a reduction of the size of the apparatus. A large power consumption means that a high density packaging cannot be achieved in the apparatus. This is because the heat cannot be sufficiently dissipated with such high density packaging. Accordingly, high density packaging is not possible, that is, the size cannot be reduced.
The ATPC function has been proposed and put into practical use in order to deal with this problem. For this, the transmission power generating stage in the radio transmitter apparatus is made able to operate in either of a high power mode or a low power mode. Due to this, the radio transmitter apparatus is made able to be operated in the low power mode in most hours during which good communications conditions are secured and is selectively operated in the high power mode for only the few hours during which the communications conditions become the worst. Thus, the above problem is solved by this APTC function.
The present invention relates to a radio transmitter apparatus provided with the above ATPC function in addition to the general ALC circuit.
2. Description of the Related Art
In a radio transmitter apparatus, the effect of ATPC by an ATPC unit is exhibited when the transmission power level in the low power mode has become sufficiently low. This is because, as already mentioned, the aim of ATPC is the reduction of the power consumption by taking note of the fact that good communications conditions are secured in most hours. This being so, it can be said that, when the transmission power level in the low power mode is sufficiently low compared with the transmission power level in the high power mode, that is, the larger a level difference between the transmission power levels in the high power mode and the low power mode, the larger the effect by ATPC.
However, even if it is attempted to set the level difference large, in actuality, not so large a level difference can be realized. This will be explained in detail later. Note that the detector forming one of the principal components of a radio transmitter apparatus is usually comprised of a Schottky diode.
In order to increase the effect of the ATPC described above, where the above level difference is increased from xcex94p to xcex94P, as already mentioned, inevitably, also the dynamic range is increased from d to D.
When the level difference is increased to the above xcex94P for example, however, it enters a nonlinear region of characteristics in a first active region and the detection sensitivity of the detection voltage becomes bad. For this reason, the required stable ALC function ends up no longer being achieved.
On the other hand, in a second active region, an excess input is applied to the Schottky diode and the diode is liable to be destroyed.
In this way, there is a problem that, even if it is desired to increase the level difference xcex94p to xcex94P, in actuality, there is the above constraint of the dynamic range, therefore it cannot be realized.
Accordingly, in consideration with the above problem, an object of the present invention is to provide a radio transmitter apparatus enabling the increase of the level difference without the danger of the destruction of the Schottky diode and while maintaining a stable ALC function.
To attain the above object, according to the present invention, there is provided a radio transmitter apparatus including a transmission power generating stage (11), a detection unit (13), an ALC circuit (16) performing automatic level control, and an ATPC unit (17) performing automatic power control for operation in either of a high power mode or a low power mode, wherein the detection unit (13) is configured by a first detection unit (13L) which becomes effective in the low power mode and a second detection unit (13H) which becomes effective in the high power mode.
Thus, by the ATPC function, a radio transmitter apparatus performing an ALC operation by selecting either of the high power mode and the low power mode and capable of further increasing the level difference of the transmission power levels in different modes while supplying a stable detection voltage to an ALC circuit without destruction of a detection diode is realized.